I would highly advise against manually fragging a BTA. They tend to split on their own once they get big enough or in response to environmental changes.
I recently transferred my BTAs into a new tank and they split like crazy.
You can also try to feed the BTA 3+ times a week. Seems to stress them enough to get them to split.
Swings in temp and salinity can cause them to split as well.
I find it best to just let them split on their own.
Woah. I would really advise against that. 3.5 inches is small for a BTA, it's still a juvenile, they get like a foot. Your chances of success are really slim. Btas are not corals, they split on thier own- sometimes- after a long process of creating 2nd mouths, tissues etc. There are people who do this but you are better off encouraging it to split by feeding a ton and offering great parameters, and letting it grow up so it can handle it.
I should have mentioned that I am trying to propagate them in a special propagation tank that will only hold these specific nems. I got the idea from a BRSTV video. I really like the idea of propagating or breeding animals, otherwise, I would not attempt to frag.
Just because it was done, or you can, doesn't mean you should. I doubt the advise given in this thread will help sway you but in case it doesn't let me add to what has already been provided.
RBTA's, or any BTA for that matter, will naturally propagate as Mother Nature intended. It may not be at your speed or pace but it will happen naturally. I started with one in a 29 gallon bio cube and within a year had 2. Few months later 3. Moved one to another 29 gallon biocube and it split. Now I'm at 4 and tired of managing 2 tanks so combine them both into a 40 breeder. About a year goes by and I'm at 6. Then 7, then 9, give a few away, then said enough and planned an upgrade. 210 gallons later I'm at 12. I thought I had it bad then started to look at others and I saw a few that split so many times naturally (no feeding, healthy tank, etc) that they became plague proportions much like Xenia.
My point in this is is two fold. You can't speed up Mother Nature. Just because you can doesn't mean you should. More importantly is if you are considering this for residual income BTA's are pretty common regardless of what people call them. Personally I wouldn't attempt a forceful split. There really is no reason unless you are being impatient and that is a bad trait to have in this hobby (not directed at you just a general observation in our hobby).
Whatever tank you are putting them in isnt really the main point. Slicing a small young nem in half is just a poor idea with low chances of success. You can try stressing them into splitting or just maintaining a helathy tank and waiting. If your goal is to sell them, you should research correct propagation methods to encourage them to split, bc slicing up babies will probably just cost you more as you'll end up with dead nems. If you just want a nem tank for yourself- why not just go buy some more and spend your time seeing if you can get the correct parameters in the tank so they propagate naturally.